1. Field of the Invention
The present invention relates to photolithography technology, more particularly to an inspection method for performance of a projection optical system of an exposure apparatus and to a photomask used for the inspection.
2. Description of the Related Art
A minimum line width and a minimum period of a pattern constructing a semiconductor device have decreased year by year. A minimum resolution line width required for an exposure apparatus resolving the pattern of the semiconductor device is currently about 100 nm or less. A minimum resolution period of the exposure apparatus is determined by an exposure wavelength λ and a numerical aperture (NA) of a projection lens. In order to form a finer pattern, it is satisfactory that the exposure wavelength λ is shortened and the NA is increased. Thus, shortening of a wavelength of exposure light has been achieved. An argon fluoride (ArF) excimer laser exposure apparatus (λ=193 nm) has been put into practical use in recent years. In addition, development of a fluorine gas (F2) excimer laser exposure apparatus (λ=157 nm) has been underway aiming for practical use in a few years. The types of optical materials usable as a photolithography lens with a wavelength of 193 nm or less are limited. In the current technology, fluorite (calcium fluoride single crystal) and fused quartz can be used for light with a wavelength of 193 nm. However, for light with a wavelength of 157 nm, only fluorite is usable.
One of the phenomena emerging as a problem in using a lens is birefringence. A refractive index is a physical quantity controlling a refraction angle and a phase velocity of light. Thus, when a projection lens shows birefringence, imaging characteristics of the projection lens are changed depending on polarization states of light. When birefringence exists in the lens, an image becomes out of focus due to, for example, formation of images at different positions for every polarization state of exposure light, and image contrast and resolution performance are decreased. Thus, there is a possibility that a fine pattern cannot be formed.
It has been revealed from recent research that fluorite has a relatively large birefringence in a specific crystallographic axis direction. Even if a lens is manufactured by taking a crystallographic axis in fluorite, as an optical axis direction, which axis shows no birefringence, the lens shows birefringence as to optical paths along directions not parallel to the optical axis. As a result, the finer the pattern to be transferred, the more diffracted light is generated in the direction not parallel to the optical direction. Therefore, the more likely the lens will be influenced by the birefringence. In order to suppress the influence of the birefringence in the lens as a whole, a plurality of fluoric lenses are disposed by alternating respective crystallographic axis directions thereof. Thus, the influence of the birefringence can be offset to some extent or can be prevented from being focused on a specific optical path. However, even in the above case, the current technology has difficulties in completely suppressing the-influence of the birefringence.
The birefringence of lens materials used in the projection optical system of the exposure apparatus deteriorates imaging performance. Therefore, the birefringence has to be suppressed. However, merely observing the deteriorated state of the image does not enable determination of whether or not the deterioration is caused by the birefringence. As the patterns of the semiconductor device become fine, it is necessary to use a lens with small birefringence in the projection optical system. Moreover, a method for inspecting whether or not birefringence is suppressed as much as possible, apart from other factors of image deterioration, is necessary.